From the July 2009 issue

Could there be large amounts of water in deep aquifers on Mars?

Jon Kutz, Lake Crystal, Minnesota
By | Published: July 27, 2009
Surface Water
These pictures introduce the basic features of a martian gully. The figure on the left is an example from Mars, the figure on the right is a gully on Earth. In the Earth picture, rain water flowing under and seeping along the base of a recently-deposited volcanic ash layer has created the gully. For Mars, water is not actually seen but is inferred from the landforms and their similarity to examples on Earth.
JPL/NASA/Malin Space Science Systems

There is good circumstantial evidence that abundant underground ice and, possibly, liquid water exist on Mars.